How do I calibrate exposure to match a gray scale?
To create a Gray Chart is quite easy:
Give the object a diffuse shader and set the albedo to 0.18 (Meaning a reflectance of 18%)

If you render the image and left click on the UV/Image editor, you can sample where the scene referred values are, and how those are being transformed into display referred values by the color transforms set in the color management section. (For blender 2.8 right click on the image and the sampler window will appear)

The numbers that move at the bottom of the screen are (From left to right) pixel coordinates, Z depth value, then Scene Referred RGBA values, Then the RGB values that follow the CM (CM stands for color managed) are the Display Referred values, those of your final image.

Sampling will help you determine how the light is affecting your scene and how those values are being transformed on the final image.
So how are the scene referred values being transformed to display referred ones? It all depends on the settings on the color management.
For filmic blender, mid gray would be a scene referred value of 0.18, and it would get mapped to a value of 0.5 in display referred values. (sRGB tends to end up with display referred 0.5 at around 0.2 scene Referred)
If you are using Filmic Log encoding in as your view transform, you can also enable the False Colour Look. That will give you an interactive way to judge the &quot;exposure&quot; of your scene. Mid gray (in display referred values) will show as gray. Darker areas will be represented as blueish-purple and black, and bright areas as red, and eventually white once the values are too large.

Note that false colour should only be used with the filmic blender log encoding views. Using other view transforms will not result in an accurate representation.
For more details read the github page of blender render: https://github.com/sobotka/filmic-blender
So the same basic cube would look something like this in false colour:

Another example:
This scene (the cube is set with a reflectance of 18%):

Woulld look like this in false colour.

To understand how filmic blender works read:
Render with a wider dynamic range in cycles to produce photorealistic looking images